Portland cement clinker is basically composed of four crystalline stages: alite (tricalcium silicate): 3CaO.SiO2 (C3S), belite (dicalcium silicate): 2CaO.SiO2 (C2S), tricalcium aluminate: 3CaO.Al2O3 (C3A) and tetracalcium aluminoferrite: 4CaO.Al2O3.Fe2O3 (C4AF). The latter is the only one colored; and, therefore, the only one responsible for the characteristic color of the ordinary gray Portland cement.
The ordinary Portland cement is the product from the ground Portland cement clinker and a setting regulator, which traditionally has been gypsum (CaSO4.2H2O).
Processes and plants used for manufacturing white Portland Cement (P.C.) clinker are widely known in the art. Generally, the manufacturing process for White P.C. clinker differs from that for Gray P.C. clinker in the following basic aspects:
(1) Chemical: To obtain a White P.C. clinker, the total Fe2O3 content from the raw materials for the White P.C. clinker is controlled and must be less than 0.5% by wt. This implies eliminating the main melting agent (flux) from the gray P.C. clinker. Consequently, the liquid stage is essentially formed only from calcium aluminates, which will crystallize after a cooling process as C3A. This thus eliminates formation of the C4AF solid solution; which is responsible for the gray P.C. color.
(2) Sintering: Eliminating the main flux found in gray Portland cement clinker, results in white P.C. clinker; which with less flux consequently increases over all the temperature for the liquid stage; from 2440.4° F. (1338° C.) for a gray P.C. clinker to 2642–2678° F. (1450°–1470°) for a white P.C. clinker. This thus requires a higher fuel consumption during sintering as compared to the gray P.C. clinker sintering process.
(3) Cooling: In the manufacturing process of white P.C. clinker, typically the clinker cooling process is more difficult than for the cooling process in the gray P.C. clinker manufacturing process, where one is trying to stabilize most of the iron on a reduced state (Fe 2+), that is less chromophore than (Fe3+).
Despite the differences above-mentioned between both types of Portland cement clinker manufacturing processes, basically the two Portland cement clinkers are generally constituted by the same main mineralogical stages: C3S, C2S, C3A and, exclusively for Portland cement gray clinker, C4AF.
Advantages of a white P.C. over a gray P.C., are basically their white color, for which it is widely used for exposed structures and texturized elements, which can be white or colored. It is also common that white Portland cement develops better compression strength due to a high content of C3A, as compared to a gray Portland cement. A disadvantage of a white Portland cement is its low resistance to sulfates attack, mainly due to the high content of C3A, that reacts with environmental sulfates and produces ettringite when mortar or concrete is hardened, causing fissures or cracking.
On the other hand, in the state of the art, it is known that sulfur coming from traditionally used fuels for the cement industry, such as gas (non-sulfur containing), carbon (1–2% S), fuel oil (2–4% S) and low sulfur content pet-coke [petroleum coke] (<5% S), is fixed on clinker in the form of anhydrite, CaOSO3 (C{hacek over (S)}), and in the presence of alkali leaching it forms alkaline sulfates: Na2O.SO3 and K2O.SO3. Said anhydrite and alkaline sulfates fixing on the clinker is a desirable factor since up to now it has represented the only way of extracting sulfur from the interior of the furnace, avoiding blockages and obstructions of the kiln itself, as well as the emission of SOx to the atmosphere.
In this way, sulfur, being semi-volatile, forms a cycle within the furnace, which re-concentrates and once such sulfur reaches a critical SO3 evaporating concentration, that is associated with clustering in the kiln which makes kiln operation difficult and unstable, thereby reducing its production (and can even form blockages serious enough to result in a stoppage of the kiln itself).
On the other hand, fuel-oil cost is inversely proportional to its sulfur content. For this reason, high sulfur content pet-coke (S>5%) represents an opportunity for the cement industry due to its availability at a lower cost.
Unfortunately, the use of such fuel is difficult, since it requires special care for operation New methods have long been needed that could ease use without detriment to the furnace operation continuity. Currently, the use of the pet-coke as fuel, in one hand, generates the necessary heat to maintain the process temperature; and on the other hand, it produces a higher amount of SO2, which can become large enough to produce blockages in the pre-heater and form rings in the rotating kiln. This causes operational troubles, reducing its efficiency and, in more serious cases, actually interrupts operation.
On one hand, and particularly in relation to the Portland cement clinker, in the state of the art there are numerous efforts aimed to facilities, equipment and/or processes design, for the use of solid fuels with a high sulfur content, in order to solve the problems associated with SO2 formation and accumulation. However, most of processes and/or plants that manufacture Portland cement clinker and use a solid fuel with a high sulfur content, present certain disadvantages such as complexity of processes and equipment, as well as high operational costs. Examples of such efforts are disclosed in, for example, the U.S. Pat. No. 4,465,460 entitled “Cement clinker production” issued to Paul Cosar on Aug. 14, 1984; U.S. Pat. Nos. 4,662,945 and 4,715,811 both entitled “Process and apparatus for manufacturing poor cement clinker in sulfur” issued to Thomas R. Lawall on May 5, 1987 and in Dec. 29, 1987 respectively; the U.S. Pat. No., 6,142,771 entitled “Cement clinker production control using high sulfur content fuel within a rotating kiln with a Lelep-Lepol displaceable grid through the sulfur final analysis in the final product”, issued to Joseph Doumet on Nov. 7, 2000; Chinese Patent No. 1,180,674 issued to Wang Xinchang et al on May 6, 1998 entitled “Method for producing high quality cement using pet-coke with a high sulfur content”.
A recent effort to solve problems associated to the use of high sulfur content coke, is disclosed by Mexican Application No. PA/a/2001/007229 entitled “Method of producing cement clinker using high sulfur content pet-coke” filed on Jul. 13, 2001 by Trademarks Europa, S.A. de C.V., which is a subsidiary of the CEMEX group (see also the corresponding U.S. Pat. No. 6,599,123, issued Jul. 29, 2003). In this latter application is discussed a method to produce cement clinker that allows a more economic and efficient use of fuels with a high sulfur content such as pet-coke and that minimizes problems associated with blockages and incrustations due to the high concentration of SO2 and/or SO3 in the system. In this document is described the production of a high quality cement clinker that does not require addition of additives to improve its final physical properties.
On the other hand, there is another group of scientists and technologists who have focused their efforts on reducing Portland cement clinker sintering temperatures, through chemical modifications (of raw mix or raw meal), and have frequently used non-traditional mineralizers and fluxes. Examples of this group's efforts are generally described in the U.S. Pat. No. 5,698,027 entitled “Method and Plant for manufacturing mineralized Portland cement clinker” granted to F.L. Smidth & Co., which is related to a mineralizer such as gypsum, fluorite, etc., as a control parameter for preventing or reducing problems associated with the rotating kiln operation; the Spanish Patent No. 542,691 “Process for obtaining white clinker with a low fuel consumption using fluorite and sulphates as raw meal components”, describes a process for producing a clinker composition which is formed under lower temperatures as compared to traditional temperatures for the Portland clinker manufacturing, forming a new liquid phase called fluorelestadite: 3C2S.3CaO4.CaF2. However, said cements present some problems with strength development, over all, at early ages (1 and 3 days), even when strength increases at long ages (28 days and more), as compared to strength developed by common Portland cement.
There is a third group of researchers who have found non-Portland clinker cement compositions, based on the formation of a phase rich in sulfur, calcium sulphoaluminate: 4CaO.3Al2O3.SO3 (C4A3{hacek over (S)}). Said calcium sulphoaluminate based cements exhibit an accelerated development of initial strengths compared to Portland cement, due to the (C4A3{hacek over (S)}) hydration to form ettingite. Examples of this branch of development are U.S. Pat. No. 6,149,724 to Poo Ulibarri et al, granted to CEMEX in 2000, or the corresponding Canadian Patent No. 2,193,339 and European Patent No. 0 812 811.
In 1994, U.S. Pat. No. 5,356,472 “Portland cement clinker and Portland cement”, to Ivan Odler, disclosed a method for manufacturing gray clinkers under low forming temperatures, for example between 2102° F.–2462° F. (1150° C.–1350° C.). This clinker was formed by C3S, (C4A3{hacek over (S)}) and preferably also C4AF phases and practically without C2S and C3A, if to the raw mix containing CaO, SiO2, Al2O3, and Fe2O3 was added an inorganic additive containing SO3 and other inorganic additive containing fluorine. Cement was obtained with strength comparable to that of conventional gray Portland cement, when the cement was prepared with a clinker composition of 80% C3S, 10% C4A3{hacek over (S)} and 10% C4AF.
However, no prior art document is related to the main objective of the present invention; which is to produce a family of mineralogical compositions of white cement clinkers, formed by C3S, C2S, C4A3{hacek over (S)}, C3A, C11A7.CaF2, C{hacek over (S)} phases, without the presence of C4AF, with a high capacity to fix sulfur derived from pet-coke as a sulfur source [which fixation enables the use of a high sulfur content fuels (>5% S)], using the conventional infrastructure of cement plants, for the production of white portland cements with compression strength similar to or even greater than the strength of conventional white Portland cement.
Therefore, it is an objective of the present invention to provide new clinker and white portland cement compositions with high capacity of fixing sulfur coming from pet-coke with a high sulfur content, used as fuel.
Another objective of the present invention is to provide new clinker and white Portland cement compositions exhibiting low fuel consumption in its manufacturing process and having fast setting developing increased compression initial strengths.
It is further another object of the present invention to provide new clinker and white portland cement compositions using the conventional cement Plants infrastructure.